Phase and frequency modulation system



Patented-Sept. 12, 1944 'r f orme-E PHASE AND FREQUENCY MoDULATIoN SYSTEM Charles William Earp, London,

England, assignor to Standard Telephones and Cables Limited,

London, England, a British company Application October 2, 1942, Serial No. 460,578

vInl Great Britain April 25,

6` Claims.

The present invention relates to an-arrangement for the conversion of frequency modular tion tophase modulation and vice versa, andthe application thereof to the` productionof a frequencymodulated. wave from a frequency stable oscillator such as a crystal controlled oscillator.

An ultimate object of the invention is to'provide a simple and 'economic arrangement for producing a frequency modulated wave of large excursiOn, that is, a large range; between the limits of` frequency variation, from a frequency stable oscillator such as a crystal controlled oscillator.

f, According to one feature of the invention,` an arrangement" for the conversion of frequency modulation to phase modulation of stable frequencycomprises ra first modulator to which the frequency modulated wave and the stablefrequency are'fed and a second modulator to which are fed the frequency modulated waves and one of the side bands in the output fromsaid first modulator, the paths between the source of phase modulated waves and said second modulator being so'designed as to have a linear frequency versusdifferential phase shift char- `acteristic, the-output of said second,v modulator bfeing passedI through a filter designed' to pass 'said stable'frequency.

From anotlier'- aspect of this feature ofthe invention anfarrangement for the conversion of a frequency modulated wave to a phase modulated wave-islcharacterised.in that a beat frequency is produced between a stable frequency `and the frequency modulated wave, the said beat frequency being given a phase shift in accordance with its deviation froma mean frequency and` then combined with the original frequency modulated wave to produce a phase modulated wave of said stable frequency.

According to another feature of the invention an arrangement for the conversion of a phase modulated wave to a frequency modulated vwave comprises a modulator and a .circuit arrangement for producing a frequency linearly variable in accordancewith the phase of said phasemodulatedv wave; said phase modulated wave and -said variablefreguency being fed to said modalator f or producing theV frequency modulated wave, said circuit arrangement comprising a second modulator to which are fed one of the sidebands of the output of the first-mentioned modulator and the unmodulated frequency of the phase modulated wave, the beat frequency in the output of saidV second modulator being fed to the input of said first modulator, the feed-back path together to a rst modulator for producing in the output thereof the frequency modulated wave, the arrangement for producing said variable frequency comprising a second modulator to which are fed one of the side-bands of ,the

output of the rst modulator and the said stable frequency, the beat frequency in the output'of said second modulator being fed to the inputof said first modulator, the feed-back path of said first modulator :thus formed being designed to have a total `phase delay .linearly variable with frequency. l.

In order to attain the ultimate object of the invention, the. phase modulated wave of stable frequency isr first produced-by the modulating wave and the phase modulated wave is then converted to a frequency modulated wave by an arrangement according to the above features of the invention.

The invention will be more clearlyaunderstood 4 from the following description taken in conjunction with the accompanying drawing which shows schematically in l Fig. 1 a circuit arrangement for Aconverting a frequency modulated wave to aV phase modu-` lated wave, and

In Fig. 2 a circuity forproducing a frequency modulated wave and utilising the arrangement shown in Fig.v 1`for first producing a phase modulated wave.

Referring to Fig. 1 of the drawing, a source of frequency modulated waves is shown at A,`

the normal unmodulated frequency being f and the frequency excursion being represented by in.

The wave energy from A is fed along two paths,4

namely directly to modulator M2 and to modulator` MI which is supplied with acarrier wave of stable frequency F from the oscillator O.' The output ofMll is fed to a filter XI in order to select one sideband of the modulation, forinstance (Fi-f) which' is then passed to the modulator M2. A delay device is represented at DI lbut this should be understood to represent geni for. producing a phase MI and M2.

eraliy the circuit elements producing thev total delay over the path from MI to M2, the phase shift being linear with frequency. In actual pracltice a separate delay network may not necessarily be required since the requisite delay may be introduced by proper design of the filter XI automatically. A well designed'filter giving a flat frequency-amplitude characteristic over the pass-band provides the necessary linear phase shift with frequency and has all the characteristics of a simple delay device for all frequencies contained within the pass-band of frequencies.

There is thus fed to the modulator M2 vtwo waves differing in frequency by F which ,is selected by a final filter X2. The amount of phase modulation of the output from M2 or lter X2 is determined by the differential delay between the two signal paths between A and M2, and delay devices may be inserted in the direct path from Ato M2, between A and MI or between The delay network DI (or XI) is such that a linear'r phase shift of 0 radians penn cycles is introduced inthe path from Ml to M2 and may, of course, be comprised bythe filter XI. If the frequency of the signal increases fromA f to (f-l-A) the phase distortion `produced by the delay network will be advanced by radians and consequently the output of F from M2 will also be advanced by 0 radians. Similarly any other frequency change between +A and A will produce Fig'. 1, that is, a source of phase modulated waves is substituted for the wave source A in Fig. 1. `This is shown in Fig. 2 by the rectangle PM. In Fig. 2, O is lan, oscillator of stable frequency F, whichv may forinstance be crystal controlled. The output from O is phase modulated in PM and the outputfrom'PM is fed to a modulator M3. Output from the oscillator O is'also fed to the modulator M4 where it is modulated by one of Vthe side bands [Fifinll whose frequency is dependent upon the phase'modulation and isfobtained as described later via a filter X4 of mean pass frequency (F-l-f) or (F-f) The output of M4 contains the frequency (fj- A) which is passed by the 1 filter X3 of mean-pass frequency f and fed to the Vmodulator' M3, which asalready stated is also supplied with the phase `modulated frequency F. The output from M3 thus includes the side 6 bands FUM yone of which, e. g. F-(fiA) vis passed by the filter X4 and isthe frequency modulated wave required and is of mean frequency F f in the example. Part of the outputof X4 is, as already stated, fed back to the modulator M4. A delay network D2 is shown between X3 and M3 but, as in the case of Fig. 1 this represents ciently great oscillations are set up and these must be of mean frequency f in order to pass' through filter X3 and (F-f) in the example to ship for oscillation round the loop has now been upset and the frequency f will rise or fall to accommodate itself to the new conditions. For instance, if the phase of -the injected frequency F into M3 is advanced with respect to the direct injection of the same frequency from O into M4,

elements of the feedback path (X4-. M4-X3) 0f M3 which produce phase shifts. A phase delay network may be used to make the'total phase shift of the feedback path linear with frequency or the filters X3 and X4 .may be properly designed in known manner to carry out this funcoscillations can only be maintained in the loop if the frequency f is increasedto (f-l-A) and the frequency (F-f) to lF-(f-|A)l. Oscillationon these new frequencies is quite stable.' I

In certain cases it may be necessaryto initiate the loop oscillations by injecting into the feedback circuit an oscillation of frequency fo lying within the limits ofthe frequency modulationof F-f (or F-l-f) An oscillator OS of frequency fo is shown'in Fig. 2, coupled up to the modulator M3 by means of a switch S. When the switch S is closed the starting oscillator OS feedsV the modulator M3 with the frequency fo for example, and the output from M3 is then (F-fo). If frequency F4-fo were used it would be introduced into the circuit at or between the output of M3 and the input of M4.

Taking the case when frequency ,fo is introduced into M3 it beats with frequency F from the phase modulation source PM, to produce frequency F-fo which is passed to M4. 4Here frequency F-fo beats with frequency F directly fed from the oscillator O to produce the difference frequency fo which is passed via the filter X3 and delay network D2 to modulator M3, 'reinforcing the initial oscillation from` the vstarting oscillator OS.V The starting 'oscillator may now Ibeswitched out of circuit, and the oscillations continue.' The oscillator O at frequency F thus produces a frequency (F-,l in the output of the filter X4A and a frequency ,f in the forward path from M4 to M3. Supposefnow the phase of the frequency F injected into'll/IS` is advanced with respect tothe wave directly injected into modulator M4, due to its phase modulation, and

vas previously stated the correct phase relation-` ship for maintaining the oscillations at frequency f and (F-fl has been upset and oscillations can only be maintained by an increase of frequency from. f to f-l-A and from (F-fl to (F-f-A). Similarly a retardation of the phase of F fed to M3 produces a change in the loop frequency from f to (f-A) and from (F-f) to (f-f-l-A).

Thus a phase modulation of i6 on the oscillator O .produces two frequencymodulated waves (fi-A) and [F-(fiml. j`

If the phase modulation is linear, since the time delay network gives Ilinear phase 4distortion with frequency, the output wave is linear in frequency with the original modulating wave.v

What is claimed is:

1. A system for producing a frequency modulated wave-of stable'mean frequency which comprises a source ofA stable frequency,'means to phase modulate the frequency from said' source,

a first modulator, means `to deliver the phase modulated wave to said first modulator, a second j v modulator, means to impress the stable frequency from said source on said second modulator,

means to impress a portion of the output of said rst modulator on said second modulator, means to impress the output of said second modulator on said first modulator, means to provide a total phase delay linearly with frequency between said second modulator and said first modulator, and means to utilize a remaining' portion of the output of said rst modulatorbas the desired frequency modulated wave.

2. A system as defined in claim 1, in which a filter is provided in the output circuit of each modulator which will pass one side band only of the output of said modulator.

3. A system as dened in claim 1, in which a filter is provided in the output circuit of-each modulator which will pass one side band only of the output of said modulator, the filter in the output circuit of said second modulator having a linear frequency vs phase shift characteristic and is the means to provide the phase delay.

4. A system as defined in claim 1 in which an oscillator is provided to inject energy into the loop circuit between the two modulators to start the system functioning.

5. A system for the conversion of a phase modulated wave to a frequency modulated wave which comprises a first modulator, means to deliver a phase modulated wave to said first modulator, a source of stable frequency at the frequency of said modulated-wave, a second lm6duiator;rnevans*A `-to impress on said secondniodulator"ajportioiifof the energy from said vsource of 'stable frequency,A means to impress a ,portion of the output of said first modulator on said second modulator, means to impress the output of said second modulatorv on said first modulator, means to provide a total phasedelay linearly with lfrequency between said second modulator and said first modulator, and means to utilize a remaining portion of the outmodulating a portion of said wave,v beating said phase modulated wave with a wave having a frequency linearly variable in accordance with the phase of said phase modulated wave, beating a portion of the resultant modulated wave with another portion of said stable frequency wave, delaying the phase of the resultant wave linearly with frequency, utilizing said phase delayed wave as said aforementioned wave having-a frequency linearly variable in accordance with the phase of said phase modulated wave which is used to beat with said phase modulated wave, and utilizing the remaining portion of the resultant of said first beating for the desired frequency modulated wave.

CHARLES WILLIAM EARP. 

